Setting is Generic Fantasy Universe, late-medievalish tech levels, climate's probably not wholly unlike, say, northern France. Not especially dry, not especially wet, snows in winter but there's rarely more than a few feet of snow on the ground, summers get warm but not generally Really Hot.
We're in a keep/castle/fortress thingie, a somewhat rambling stone/brick/wood building (a bit like if you sort of squooshed a small town/village into one large building). It may reach as high as 3 stories in some places.
Where, in such a structure, would you put your library, assuming you knew something about Proper Care And Feeding Of Books? I'm thinking the main concerns are keeping them dry (humidity->mold), keeping the temperature relatively even, and avoiding fire/smoke. Assume that the building was built with the assumption that there would be a (smallish) library, and that said library would have the best location reasonably practical.
I'm not sure if I'm best off with an interior room (thus less temperature fluctuation) or an exterior (thus natural light). And I have no idea where you'd find the least humidity (other than "not the basement") I suspect locating one wall along the chimney for the kitchens would be a Good Thing...